In a reaction between solutions, some ions change and others do not. An ionic equation shows only the ions and particles that actually take part. Ions that are present but stay the same are called spectator ions, and they are left out.
Worked example: silver nitrate solution reacts with sodium chloride solution to make a white solid, silver chloride. The full equation is AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq). The sodium ions and the nitrate ions are the same on both sides, so they are spectator ions. The ionic equation is Ag+(aq) + Cl-(aq) → AgCl(s).
An ionic equation must be balanced for atoms and also for charge. The total charge on the left must equal the total charge on the right. In Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s) the total charge is 2+ on each side. In the neutralisation of an acid by an alkali, hydrogen ions react with hydroxide ions to make water: H+(aq) + OH-(aq) → H2O(l). The total charge on each side is zero.
To write an ionic equation, split the aqueous ionic compounds into ions, cancel the spectator ions, then check the numbers of atoms and the total charge are the same on both sides.